The cutting or removal of tissue electrosurgically using an instrument having a tip with one or more active electrodes supplied with a radio frequency (r.f.) voltage usually involves cell rupture as a result of arcs between the active electrode and the tissue being treated or, in the case of underwater electrosurgery, between the active electrode or electrodes and a conductive liquid such as saline overlying the tissue to be treated. As described in EP-A-0754437, electrode destruction can occur if sufficient radio frequency power is supplied to an electrode to cause burning or melting of the electrode material, and this can be avoided by sensing peak electrode voltage and applying feedback to reduce the applied power so as to set a maximum peak voltage. It will be understood that for a given power setting, the temperature of the electrode depends on the rate at which heat can be dissipated which, in turn, depends on such variables as the degree of tissue engagement, electrode structure, and fluid flow around the electrode. Consequently, to avoid electrode destruction the peak voltage limit must be set at a sufficiently low level to prevent damage in the worst case dissipation situations, i.e. when there is an absence of cooling fluid and/or the electrode is surrounded by tissue.
In the absence of such control, the temperature of the electrode follows an asymptotic curve as shown in FIG. 1. The saline absorbs power until the point of vaporisation is reached at time `t.sub.1 `. When the saline is vaporised, the active tip temperature rises more rapidly until, at time `t.sub.2 `. active electrode destruction occurs at a temperature of 1600.degree. C. (melting point of platinum). This destruction temperature is indicated by temperature `T.sub.D ` in FIG. 1. The time taken to reach this temperature after vaporisation occurs is dependent on both thermal capacity and thermal dissipation mechanisms. A low mass electrode heats up faster. The principal dissipation mechanism is infra-red emission and is, therefore, dependent on surface area.
Limitation of peak voltage is used, as described above, to control the applied r.f. power so as to prevent the electrode temperature reaching T.sub.D under all normal operating conditions. It will be appreciated that this limits the rate at which tissue can be removed.
It is an object of the present invention to provide a means of increasing the rate of tissue removal.